1. Field of the Invention
The present invention relates to an antenna device, a card-type communication device, and a communication terminal apparatus for use in, for example, an RFID (Radio Frequency Identification) system.
2. Description of the Related Art
These days, an RFID system is being developed as a product management system. In such an RFID system, non-contact communication is performed between a reader/writer for generating an electromagnetic induction field and an IC chip (also referred to as an “RFID tag”, “wireless IC chip”, or “wireless communication device”) attached to a product or a container and storing predetermined information therein, so that information can be transferred between the reader/writer and the IC chip. In this RFID system, an HF band (high frequency signal at 13 MHz) is generally used.
In an RFID system using an HF band, a coil antenna is used as an antenna element for each of a reader/writer and an RFID tag. However, when the coil antenna of the RFID tag is brought close to the coil antenna of the reader/writer, a state (null state) in which communication is not established at a certain point (null point) may occur even if the RFID tag and the reader/writer are located within an area where communication can be established. The reason why this state occurs is that the two coil antennas repel each other so as to shift the resonant frequency or the phase, or that, when a conductor or a dielectric is brought close to the RFID tag, the resonant frequency of the coil antenna of the RFID tag is shifted. The resonant frequency is increased if a conductor is brought close to the RFID tag, while the resonant frequency is decreased if a dielectric is brought close to the RFID tag.
With this background, Japanese Unexamined Patent Application Publication No. 2006-238398 and Japanese Unexamined Patent Application Publication No. 2010-154255, for example, disclose a technique for preventing the occurrence of a null state in the following manner. A switching device or an oscillator is connected to a coil antenna, and by sequentially changing the state of the switching device or by sequentially changing the oscillation frequency of the oscillator, the resonant frequency of the coil antenna itself is shifted. In this case, however, since the resonant frequency of the coil antenna itself is shifted, the Q factor of the coil antenna is reduced, or noise of the switching device or the oscillator is superposed on the coil antenna.